1. Field of the Invention
The present invention relates to a focus detection system which is incorporated in optical equipment such as an SLR camera.
2. Description of the Related Art
In a conventional phase difference focus detection system incorporated in an SLR camera system to serve an autofocus (AF) system thereof, a pair of object images obtained by pupil division are projected onto a pair of areas on a line sensor (line sensor array) to be converted into a pair of electric charges (electrical signals) by the line sensor, and the pair of electric charges are converted as a pair of image signals, respectively. Thereafter, a phase difference between the pair of object images formed on the line sensor is determined according to the pair of image signals, and a focus state (defocus amount) is determined based on the phase difference.
In the case where such a focus detection system is a multipoint focus detection system capable of focusing on a plurality of focus detection zones, a monitor sensor (monitor sensor array) is installed on each of a plurality of line sensors or each of a plurality of zones on each line sensor which respectively correspond to the plurality of focus detecting zones, and an integral termination operation and an amplification control (auto gain control) on image signals are carried out in accordance with one of the plurality of monitor sensors which has received an object image having the greatest brightness. Therefore, it is seldom the case that the image signal output from any line sensor reaches a level of saturation. This type of multipoint focus detection system is disclosed in, e.g., U.S. Pat. No. 6,198,506.
However, in recent years there has been an increasing tendency to provide a multipoint focus detection system with an elongated line sensor on each of a large number of focus detection zones or on aligned focus detection zones. Accordingly, installing a plurality of monitor sensors on each line sensor or each focus detection zone has become difficult due to the limited chip size of each monitor sensor.
Due to the tendency of increasing the number of focus detection zones and miniaturization of an optical system of the autofocus (AF) system, the amount of light which illuminates the light receiving surface of each line sensor tends to decrease, i.e., there is a tendency for an object image projected onto each line sensor to become dark, which increases the integration time and the operating time of autofocusing operation.